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Origin of Cuyama Basin Oils

Lee Lundell, Stuart Gordon

Geochemical and geological data suggest that there are two possible source rocks for the oil produced in the Cuyama basin: the Soda Lake Shale Member of the Vaqueros Formation and the Monterey Formation. The Soda Lake is the more likely of the two, but the Monterey cannot be ruled out.

Nine oils from the three largest Cuyama basin fields were studied. Geochemical data suggest that the oils originated from the same source rock, and they are similar to Miocene-sourced oils in other southern California basins. Source-rock analyses indicate that, at least locally, both the Monterey and the Soda Lake have good oil-source potential. For both units, total organic carbon values range up to about 5%, Rock-Eval S2 values range up to near 30 mg HC/g TOC, and hydrogen index values range up to near 600. However, although the Monterey is rich and oil-prone almost everywhere, the Soda Lake is in some places lean and gas prone. Even so, carbon isotopic data from rock extracts suggest that the Soda Lake shale is the more likely source of the oils.

The oils in the fields were probably not generated indigenously. Tmax data and burial-history analysis indicate that near the oil fields both formations are too immature to have generated significant amounts of light sweet oil. Instead, it is more likely that the oils were generated in the deep, extensional basin in the footwall of the Morales-Big Spring thrust system.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.